The determination of the presence and location of concealed underground objects, such as gas and water pipes, power cables, and telephone and CATV cables or conduits, is a necessary prerequisite to excavation and/or the laying of new lines or cables. In some applications, an underground steerable boring tool is utilized to form an underground tunnel in which cables, telephone lines, etc. are subsequently positioned. When using such a steerable boring tool, it is important to know the location and orientation of the boring tool relative to underground objects to be able to appropriately steer the boring tool and thus position the new lines or cables to avoid existing lines and cables.
One method of boring is described in commonly assigned, U.S. Pat. No. 4,953,638 (the '638 patent) filed on Jun. 27, 1988, incorporated herein by reference. As generally indicated in FIGS. 1 to 3, the method includes positioning a boring machine on the surface of the earth adjacent a selected borehole entry point. The boring machine includes facilities to axially advance and to selectively rotate a drill string. The present invention, however, is not restricted to the method in the '638 patent, but has broad applicability to other methods as well, such as positioning the boring machine in a subsurface pit. The drill string is typically in the form of a plurality of lengths of pipe which are provided with male threads on a first end and female threads on a second end so that the lengths of pipe may be interconnected together in sequence to provide a drill string. At the end of the drill string, a drill bit as shown in FIGS. 2 and 3 is provided for performing the boring operation. The drill bit includes a blade which is inclined at an angle to the axis of the drill bit string to which the bit is attached. The angled blade of the illustrated drill bit generates a non-axisymmetric resultant force as it is thrusted through the ground, causing it to deviate off a straight line path if the bit is not rotated as it is advanced.
The drill string is simultaneously rotated and advanced by means of the boring machine to establish a borehole in the earth. The drilling operation wherein the pipe is simultaneously rotated and axially advanced is continued until a change in direction of the borehole is desired, such as to avoid a known obstacle or to correct a course deviation. In order to change the direction of the borehole, the following sequence is employed:
1. The rotation of the drill string is stopped.
2. The rotational position of the drill string is oriented so that the drill bit blade is inclined at an angle relative to the axis of the drill string toward the desired new direction of the borehole.
3. The drill string is axially advanced without rotation to axially advance the drill bit a short distance or as far as possible when in difficult drilling conditions such that the blade moves the drill bit in the earth toward the new desired direction.
4. Simultaneous rotation and axial advancement of the drill string may be resumed for a short distance.
5. Sequentially repeating steps 1, 2, 3 and 4 until the direction of the borehole is in the new direction desired. Thereafter, the drill string is axially advanced and simultaneously rotated until it is again desired to change directions. To again change the direction of the borehole, the above sequence is repeated.
Referring to FIG. 1, the above-described technique will be illustrated. The boring machine is generally indicated by the numeral 10 and is shown resting on the earth's surface 12 and in position for forming a borehole 14 underneath an obstruction on the earth such as a roadway 16. Alternatively, the boring machine may be placed in a subsurface pit, as is well-known to those skilled in this technology. As shown in FIG. 1, by using the machine 10 the direction of the borehole can be changed as the borehole passes under roadway 16. This illustrates how the machine 10 can be utilized to form a borehole 14 under an obstruction without first digging a deep ditch in which to place a horizontal boring machine, and, also, without having to dig a deep ditch on the opposite side of the obstruction where the borehole is to be received.
A typical drill bit 58 is illustrated in FIGS. 2 and 3. The drill bit includes a body portion 62 which has a rearward end portion 64 and a forward end portion 66. The rearward end portion 64 includes an internally threaded recess 68 which receives the external threads 70 at the drill string forward end 56. A blade 72 is affixed to body portion 62. The plane of blade 72 is inclined at an acute angle to the axis 74 of the bit. Axis 74 is also the axis of the drill string 44. The blade 72 is preferably sharpened at its outer forward end 72A. When rotated, the blade cuts a circular pattern.
To form a borehole 14 in the earth, the operator attaches the drill pipe and drill bit to the boring machine, begins rotation of the drill pipe and at the same time, causes the boring machine to linearly advance in the travel path of the frame towards the forward end thereof. The drill bit 58, rotating and advancing, enters the earth and forms a borehole therein. As long as the bit 58 is rotated as it is advanced, the borehole generally follows the axis of the drill pipe; that is, the borehole continues to go in the direction in which it is started. When the borehole is started at the earth's surface to go under an obstruction such as a highway, the borehole must first extend downwardly beneath the roadway. When the borehole has reached the necessary depth, the operator must then change the direction of drilling so as to drill horizontally. This can be accomplished in the following way. When it is time to change direction, the operator stops drilling and rotates the drill string so that the drill bit blade 72 is oriented in a desired direction. In the situation illustrated in FIG. 1, the direction of the borehole is first changed so that instead of being inclined downwardly, it is horizontal. To effect such a change in direction, the operator will rotate the drill string until an indicator indicates that the blade 72 is facing downwardly as in FIG. 3, so as to cause the drill bit to be deflected upwardly when advanced without rotation.
With rotation stopped and the blade properly oriented, the operator causes the drill machine to move forward without rotating the drill pipe. After forcing the bit as far as possible, the operator may begin rotation of the drill bit and continue to advance the drill string for a short distance. This facilitates the turning process in some soils. The procedure may be sequentially repeated until the direction of drilling has changed to that which is desired. After the borehole has been oriented in the desired direction, such as horizontal, the drilling can continue by simultaneous rotation and advancement of the drill string, adding new lengths of drill pipe as necessary until it is again desired to change direction of drilling.
Other boring techniques are also commonly utilized. For example, in a percussive mole such as shown in U.S. Pat. No. 4,907,658 to Stangl et al. which is incorporated herein by reference, the forward or boring end generally includes an anvil which is hit by an internal striker powered by compressed air. Generally, the rearward end of the mole is connected to a whip hose which in turn is connected to a flexible air hose connected to a source of compressed air on the surface. The percussive mole can also be adapted to both push or pull pipes through the ground.
As discussed above, the orientation of the angled blade of the drill bit determines the direction in which the boring tool will advance when it is thrusted through the ground without rotation. Thus, in order to appropriately steer boring tools such as those described above in a particular direction, the orientation of the angled blade must be known accurately. Additionally, this angular orientation information must be effectively presented to the operating crew in order to permit efficient underground boring to be carried out.
The prior art contains a number of techniques of determining this angular orientation information. U.S. Pat. No. 4,714,118 to Baker et al., for example, discloses a method and apparatus for monitoring the rool angle of a boring device. The arrangement includes a cylindrical support housing and an electrical resistor element mounted concentrically about an inner surface of the housing. The resistor element forms part of an overall potentiometer which also includes a brush or contact member extending radially from and mounted to a support arm. As the boring device rotates, the resistor element rotates relative to the brush, thereby increasing or decreasing the resistance of the potentiometer. This permits a determination of roll angle in accordance with the resistance.
Another prior art roll sensor is disclosed in U.S. Pat. No. 4,672,753 to Kent. This type of sensor provides 360.degree. of roll angle indication but does not lend itself to miniaturization and is difficult to manufacture.
Another means of indicating roll angle utilizes one or more mercury switches. Such a roll sensor typically can provide only one or two position indications within 360.degree. of rotation. Therefore, actual tool face positioning for a desired steering direction must be relative to one of these positions. This is accomplished by marking the drill string and rotating the desired amount therefrom. Since the actual tool face angle cannot be measured (unless the desired tool face angle happens to coincide with a position where the mercury switch is in the ON position), errors can occur due to the incorrect marking of the reference position, incorrect amount of rotation therefrom, or from windup in the drill string. Additionally, creating a narrow ON position with a mercury switched device creates manufacturing difficulties. Further, these switches exhibit inconsistent operation when subjected to vibration or when inclined more than 10-20 degrees from the horizontal as might occur when boring down a steep hill.
Mechanical systems for determining the orientation, or roll angle, of the blade have physically marked the position of the blade on a first length of drill pipe. As each successive length of drill pipe is added, a corresponding mark is placed on the drill pipe. This process is cumbersome, time-consuming, and inaccurate.
In order to appropriately steer a boring tool in a particular direction, an operator must be alerted that the drill is not on the proper course due to obstacles. While electromagnetic locating techniques may be used to effectively map out underground obstacles such as pipes and cables, other obstacles such as rocks and tree roots are often not discernible from a visual inspection of the surface features and cannot be located using electromagnetic techniques. An operator must nonetheless be made aware of such obstacles and counteract their effect on the drill bit in order that the borehole does not deviate off the intended path and the boring equipment is not damaged. Thus, it would be desirable to provide a device which alerts an operator that the drill is being deflected off course by metallic obstacles as well as rocks and/or roots. Such a tracking or location system would allow the operating crew to determine the location and depth of the drill bit relative to a desired path for the bore and to orient the bit to maintain this path whenever deviations occur.